In the same way I have elaborated an index of trailhead modules to work on if you want to get App Builder certification, I have created a similar index for Platform Developer I. Here you have the index:

SALESFORCE FUNDAMENTALS (10%)

Describe the considerations when developing in a multi-tenant environment.

Describe how the Salesforce platform features map to the MVC pattern.

Describe the capabilities of the core CRM objects in the Salesforce schema.

Identify the common scenarios for extending an application’s capabilities using the AppExchange.

Given a scenario, identify common use cases for declarative customization.